The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Most modern vehicles have one or more of a traction control system (TCS), an antilock brake system (ABS), or some other form of advanced stability enhancement system. These systems operate with the assist of a brake system regulating pressure applied to brake calipers at each of the wheels.
The vehicle powertrain system, which may comprise the engine, transmission, prop shaft, axles, and wheels is, in effect, a system of rotary masses and torsional springs that has an oscillatory behavior. Surface (road) friction provides the counteracting torque to the powertrain-applied torque, and therefore acts as a damping force that stabilizes the system.
Wheel slip is induced under heavy acceleration or braking. At some level of wheel slip, the surface friction (i.e., grip of the road surface on the tire) is no longer able to match the applied torque (i.e., torque being applied to the tire via the wheel). This results in a reduced damping force in the system, and oscillations in the wheel develop at the natural frequency of the wheels or powertrain (typically 5 to 10 Hz). This wheel oscillation can induce vibrations that are uncomfortable to occupants of the vehicle. The vibrations, if severe enough, may also cause damage to one or more portions of the vehicle including its axle, wheels, body, etc.
Current available solutions may include filtering the ABS or TCS controlled pressure signals, adding a “dead zone” in the controls, or adding a dead zone in the sensor-measured wheel speed/acceleration signals when the wheel oscillation is detected. This will not mitigate the actual wheel oscillation, but such a system would attempt to avoid amplifying the oscillation by regular ABS/TCS actions. Other current solutions may include the reduction of engine torque. Reduction of the engine torque, however, will lead to sacrificing vehicle acceleration.
It will also appreciate that the changing frequency that occurs with wheel oscillation can vary with characteristics of the vehicle's chassis, tires, weight distribution and/or road conditions. If the frequency matches the wheel natural frequency, this oscillation condition tends to increase and worsen in magnitude during hard acceleration of the vehicle.